Extreme STOL/VTOL homebuilt

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by BBerson, Jun 18, 2008.

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  1. Apr 2, 2012 #41

    Topaz

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    Interesting. He's got a few more vids on YouTube, including a (manned?) takeoff in STOL mode using four engines.

    Anyone have more information on this project?
     
  2. Apr 2, 2012 #42

    Starman

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    This is the best extreme STOL/VTOL design.

    [​IMG]

    I'm not opinionated either :gig:
     

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  3. Apr 2, 2012 #43

    bmcj

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    Unmanned? I don't see anyone in the seat.
     
  4. Apr 2, 2012 #44

    Topaz

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    Yeah. I'm guessing they either don't have enough thrust margin to lift a pilot, or don't really trust their powered-lift control system yet. :gig:

    There's another video on their YouTube channel that looks like a manned STOL takeoff with fewer engines.
     
  5. Apr 2, 2012 #45

    Holden

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    Starman,

    I agree.

    But why is it so good?

    As I see it, it is because most of the wing is behind the prop.

    The flying pancake made the span small and the prop large. The "wing" is all behind the prop and is therefore in "dynamic" air.

    In contrast, a typical STOL has a large span and small prop. The prop only hits a small part of the wing and mostly it is confused by a non aerodynamic (lifting) fuselage.

    Lesson learned? Put most of the wing behind the prop.

    Problems in doing so? Stall, pitch moments, stability in high AOA, and sudden power off control and prop blocking, not to mention a lot of other things.


    Holden.
     
  6. Apr 2, 2012 #46

    BBerson

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    That green CGS Hawk with eight engines is exactly what I have in mind! Because the wing is fixed in flight mode, so the transition is much simpler and fail safe.

    But I would use eight electric motors, for quick and easy start/stop.

    thanks for the video.
     
  7. Apr 2, 2012 #47

    Starman

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    There are two primary reasons that this design is the best STOL, in addition to most of the wing being 'blown', and they are: 1. The props rotate counter to the wing tip vortex and so they suck air off the top of the wing and stuff it underneath the wing, so the plane rides upwards on an air cushion of it's own making. 2. The large diameter of the props give it a helicopter like ability so it can fly vertically (for a heavy fighter). In a 20mph breeze it can take off with zero ground roll.

    It is super stable, super easy to control, has a nearly non existent stall, and the two interlinked engines which will prevent most engine loss problems.

    BUT, and that's a really big but isn't it? ... if it looses all power then we don't want to talk about it. :gig:
     
  8. Apr 2, 2012 #48

    Starman

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    Now there's a good idea, except due to the reliability of electrics you can use less motors, like three or four, and I would use larger props, which will allow for smaller motors, smaller battery, etc.
     
  9. Apr 2, 2012 #49

    sachaknoop

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    Larger props, and smaller motors? I don't get it!

    Sacha
     
  10. Apr 2, 2012 #50

    Starman

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    Larger diameter = more disc area. More disc area requires lower power to produce the same thrust that a smaller disc area would, particularly at a standstill.
     
  11. Apr 2, 2012 #51

    sachaknoop

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    So a a helicopter disc size would be optimal? :)

    Sacha
     
  12. Apr 2, 2012 #52

    Starman

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  13. Apr 3, 2012 #53

    Aircar

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    thanks for bumping this thread --I had forgotten about our friend Holden and several other nascent flying car advocates --is Holden still 'in' HBA does anyone know or a PM address ?

    It is possible to share the vision about utilitarian flying vehicles that might one day capture the imagination of much more of the population and maybe even be affordable for some young people who will carry the torch for aviation into the future . 'Holden' stirs a distant memory from "catcher in the rye" (holden Caulfield) which is the prototypical novel on teenage angst and alienation --perhaps that is why he chose the moniker...

    It is also the model name for "Australia's own car" -the Holden (actually being a division of General Motors -- in trying to 'save' the precarious Australian car industry presently being almost given last rites , it is my belief that ONLY something very 'radical' can overcome the logistic and economic disadvantages compared to the rising Asian car builders --it won't be battery cars or some 'retro' styling or some other gimmick but it COULD be that the time has come to seriously address the flying car as the next logical step and possibly the only prospective solution. To enjoy the efficiencies of air travel we cannot rely upon the 'legacy' airports that are simply impossible to carry any significant amount of extra traffic and this by neccesity means that any viable flying car HAS to be what we call ESTOL (extreme STOL)hence this ability has to be incorporated into practical flying cars but seldom is (unless VTOL which then runs into the non converging weight vs power relationship )

    The desire for a 'land on a dime' but also 'go like hell' airplane is common to all designers but is constrained by the laws of physics to always pretty much the same general speed range of around three to one and rapidly shrinks when low power or economy is added to the formula --the Chance Vought XF5 "flying flapjack" discussed here never actually flew but could have very greatly expanded the speed range by applying some original thinking thanks to Charles Zimmermann doing something about his dissatifaction with the "Piper Cub' formula . Sad that it was not at least tested before being broken up on the presumption that the newly arrived jet engine would have made all prop driven aircraft obsolete (they wanted the silver in the bearings )
    I wrote to Mr Zimmerman about that concept back in the 80s and received a very nice reply and his belief in the validity of the approach still (I have a file about an inch thick on these type of aircraft --just on magazine articles and tech reports and dozens of design studies, models etc from investigating the possibilities ) Apart from the possible adaptation to a single seater I don't think that it is a viable way to a roadable aircraft -of the 'integral' type that is complete when on the road ;- and as some of the discussion suggested it is much harder to actually hover than to reach a very low forward speed but there are two conflicting criteria at work which dictate an 'optimum' minimum speed (apart from the increased gust and crosswing sensitivity with lower speeds )

    To fly slowly by means of wing lift dictates that the wing area shall grow as 1/v squared so that as the speed drops in a linear way the wing must grow exponentially (the exponent being two but actually greater to account for the weight of the bigger aircraft ) -- to hover requires much more power and to still make the same lift the power must grow as W minus 1/v squared as the wing is less and less capable of making lift .

    The usual case is that you end up with a very small wing sized for cruise which becomes a very high speed because of the high inbuilt power needed to hover --a great speed range but a widowmaker if the power fails . At least the "Flapjack" types retain a good power off behaviour (provided the props can feather ) and are virtually unstallable --whatever kind of V/STOL you are after it must at least give the pilot a reasonable chance of pulling off a survivable landing which so many of the other VTOL configurations certainly didn't .

    what I believe is that the optimum solution will be an aircraft type that (a) "morphs" in flight to get around the mismatch between the high and low speed, better ride in turbulence etc and (B) puts some of the problem onto an infrastructure in the form of ATOL -(externally) Assisted Take Off and Landing --the analogy to motor cars would be to not build bridges but require every car to be amphibious and in addition not need smooth made and drained roads but have tank tracks --in practice it is a better trade off to use some infrastructure to ease the problem for the vehicle.

    Once we appreciate this the whole design equation changes and becomes soluble.
     
  14. Apr 4, 2012 #54

    Starman

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    I guess you would need four instead of three in order to counteract torque. Or you could have two mains on the wing rotating in opposite directions and a smaller tail motor with vanes for directional control.
     
  15. Apr 4, 2012 #55

    BBerson

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    The advantage of six or more is that if one fails the others can takeover rather easily with slightly more power.
     
  16. Apr 4, 2012 #56

    bmcj

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    The CGS pilot must have been thinking about transition too. Notice that the front edge motors point up and the rear edged motors point down... I suspect that is so they can be rotated for forward thrust.



    Apparently so, since his last post was only two days ago. The link to send him a PM (through the forum) is
    http://www.homebuiltairplanes.com/forums/private.php?do=newpm&u=31.
     
  17. Apr 4, 2012 #57

    bmcj

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    Could this be the Joby hover-phase proof of concept? Or maybe a poor man's Joby?
     
    Last edited: Apr 4, 2012
  18. Apr 4, 2012 #58

    Starman

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    I think you would need at least eight in order to allow for an outboard motor failure. Six would imply having two on the center line, where they wouldn't fill in for an outboard motor failure. With eight, if an outboard one failed, then you could power down the one opposite to it to help achieve balance. I suppose that would be built in automatically into an idea motor control system.
     
  19. Apr 4, 2012 #59

    Starman

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    Of course, at first though, without looking carefully, I assumed there would be a separate thrust motor and that the vertical engines would just be turned off in flight.

    I see a couple of problems with that design. One is that the tubes that hold the motors go above the wings, which will disrupt airflow in the wrong place. Underneath would be better.
    Another thing is that during transition the front engines would be blowing downwards on the wing creating a lot of negative lift at the same time as the rear engines are still generating upwards lift. It would require a rather sudden transition to fix the resulting nose dive scenario.
     
  20. Apr 4, 2012 #60

    BBerson

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    No and no.Joby would go with electric not piston engines. And Joby is a tilt wing. This CGS prototype VTOL is a tilt-prop (or maybe fixed lift prop), much better in my opinion, for the reasons I mentioned.
     

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